1. Field of the Invention
The present invention relates to near-field antenna and, in particularly, to spatially selective antennas capable of selectively communicating with a targeted transponder from among a group of adjacent transponders.
2. Description of Related Art
Radio frequency identification (“RFID”) transponders, either active (e.g., battery-powered, -assisted, or -supported) or passive, are typically used with an RFID reader or similar device for communicating information back and forth. In order to communicate, the reader exposes the transponder to a radio frequency (RF) electromagnetic field or signal. In the case of a passive transponder, the RF electromagnetic field energizes the transponder and thereby enables the transponder to respond to the reader by re-radiating the received signal back and modulating the field in a well-known technique called backscattering. In the case of an active transponder, the transponder may respond to the electromagnetic field by transmitting an independently powered reply signal to the reader.
Problems can occur when interrogating multiple adjacent transponders regardless on whether the transponders are passively or actively powered. For example, an interrogating electromagnetic signal may activate more than one transponder at a given time. This simultaneous activation of multiple transponders may lead to collision or communication errors because each of the multiple transponders may transmit reply signals to the reader at the same time.
Several collision management techniques commercially exist for allowing near simultaneous communication between multiple transponders and a single reader while reducing communication errors. However, such collision management techniques tend to increase system complexity, cost, and interrogation time. Furthermore, such techniques are often “blind” in that it cannot determine what transponder or transponders are responding out of a plurality of transponders near the antenna of the reader. For example, in a printer-encoder device, the device would not know whether the reader was communicating with a transponder embedded in a label closest to the printhead or not.
Another method of preventing multiple transponder activation is to isolate transponders from one another. For example, devices or systems may employ an RF-shielded housing or anechoic chamber for shielding the adjacent and non-targeted transponders from the electromagnetic field. In various applications, transponders individually pass though a shielded housing for individualized exposure to an interrogating RF electromagnetic field. Unfortunately, RF-shielded housings add cost and complexity to a system and limit the type (i.e., form-factor) of transponders that can be processed by the system. Furthermore, many systems are limited with regard to space or weight and, thus, cannot accommodate such shielded housings.
The challenge of avoiding multiple transponder activation may be especially acute in some applications. RFID printer-encoders are one example. RFID printer-encoders are devices capable of encoding and printing a series or stream of labels with embedded transponders. The close proximity of the transponders to each other, during processing, makes targeting a particular transponder for encoding purposes problematic. Moreover, the space, cost, and weight restrictions associated with such devices, among other factors, make collision management techniques or shielding components for alleviating multiple transponder activation less than desirable.
Another example of an application in which multiple transponder activation may be acute is a conveyor system.